It is commonly known that carbon black is used in order to increase the reinforcement of a rubber composition.
Recently, the level of quality of carbon black has been higher, and carbon black having a larger specific surface area is used to improve the reinforcing property such as wear resistance.
That is, the use of carbon black having a specific surface area of more than 130 m.sup.2 /g as measured by the nitrogen adsorption method (N.sub.2 SA) is more and more increased. It is necessary to disperse the carbon black homogeneously in a rubber matrix to achieve a sufficient effect of reinforcement due to its own property.
However, it is difficult to disperse it homogeneously because of strong adhesion between carbon black particles. As a result, the reinforcement effect of the carbon black having a large surface area is deteriorated.
Further, in this case much energy is needed to disperse the carbon black homogeneously. This degrades the efficiency of productivity in tire manufacturing. In addition, the viscosity of the rubber composition increase extremely and this phenomenon also lowers the efficiency of the productivity of the tires remarkably.
In order to satisfy the above described demands, improvement in the properties of rubber compositions has been attempted by treating the surface of the carbon black.
Japanese Patent Publication No. 5643/1970, No. 24462/1983 and No. 30417/1968 disclose carbon black which is treated with oily substances or high molecular substances so as to lower the cohesive energy between carbon black particles.
However, in such methods the effect of the dispersibility of carbon black having a N.sub.2 SA of more than 130 m.sup.2 /g is still poor.
That is, when the carbon black, especially having a N.sub.2 SA of more than 130 m.sup.2 /g is mixed with rubber, a layer, called a gel, is easily generated.
It is believed that the carbon gel is generated at the surface of the carbon black in the beginning of the mixing of the particles of carbon black and thereby lowers the dispersibility of the carbon black.
Further, it is known that the resulting rubber compound has an increased viscosity which make the kneading workability difficult. This phenomenon is much more remarkable when using carbon black having a N.sub.2 SA more than 130 m.sup.2 /g.
The inventors have made various investigations with respect to carbon black in a rubber composition used in tires and other rubber articles, and found out that when carbon black having a large specific surface is treated with a specific amine or quinoline compound, both the reinforcement property and the processing of the rubber composition is improved.